Briggs–Rauscher reaction
This slowly fades to colourless and the process repeats, about ten times in the most popular formulation, before ending as a dark blue liquid smelling strongly of iodine.
The first known homogeneous oscillating chemical reaction, reported by W. C. Bray in 1921,[1] was between hydrogen peroxide (H2O2) and iodate (IO−3) in acidic solution.
[2] The BZ reaction is suitable as a demonstration, but it too met with skepticism, largely because such oscillatory behaviour was unheard of up to that time, until Anatol Zhabotinsky learned of it and in 1964 published his research.
[3] In May 1972 a pair of articles in the Journal of Chemical Education[4][5] brought it to the attention of Thomas Briggs and Warren Rauscher, two science instructors at Galileo High School in San Francisco.
The reaction is "poisoned" by chloride (Cl−) ion, which must therefore be avoided, and will oscillate under a fairly wide range of initial concentrations.
All of the components present in the residual mixture are of environmental concern: Iodate, iodine and hydrogen peroxide are strong oxidants, the acid is corrosive and manganese has been suggested to cause neurological disorders.
[10] A simple method has been developed employing thiosulfate and carbonate – two inexpensive salts – to remove all oxidants, neutralize the acidity and recover the manganous ion in the form of manganese dioxide.
Stirring the solution throughout the reaction is helpful for sharp colour changes; otherwise spatial variations may develop (see § Videos).
By replacing the starch with a fluorescent dye, Weinberg and Muyskens (2007) produced a demonstration visible in darkness under UV illumination.
Compared to existing assay methods, this procedure is quick and easy and operates at the pH of the human stomach.
Meanwhile, process A slowly generates the intermediate iodide ion out of the free iodine at an increasing rate proportional to its (i.e. I2) concentration.
Thus, eventually the concentration of free iodine (and thus iodide) falls low enough for process B to start up again and the cycle repeats as long as the original reactants hold out.
